Chemical safety: peroxide formation in 2-propanol

Two years ago we experienced an explosion in our lab at the end of 2-propanol distillation. Luckily, it was a small-scale distillation. To elucidate what happened, we prepared several samples of 2-propanol and kept them on a shelf away from direct sunlight at room temperature.

b) 2-propanol refluxed for 1.5 hours with magnesium chips and a small amount of iodine, followed by distillation; the first 10% of distillate was discarded.

c) 2-propanol from sample (b) after additional distillation; fresh 0.3-nm molecular sieves were added to the final sample to 20% by volume.

We replaced the air above the 2-propanol every two months by flashing the interior of the bottles with an airstream. After 24 months, we tested the content of all containers for peroxides using the standard iodide method (ASTM International 2008, DOI: 10.1520/e0298-08).

We found peroxides only in sample (b), which had a peroxide concentration of approximately 0.004 mol/L. We suggest that diisopropyl ether, a well-known peroxidizable substance, formed during refluxing of 2-propanol with magnesium. Diisopropyl ether boils at 69 °C, which is 14 °C lower than the boiling point of 2-propanol, and should be removed by distillation. Our results indicate that the first distillation performed without a Vigreux column was not sufficient [sample (b)], while an additional second distillation led to complete removal of all of the formed peroxidizable substance [sample (c)].

Prior to this incident, we were not aware that primary and secondary alcohols are peroxidizable. We hope this report is a reminder to the chemistry community that they are and that distillation procedures should take the hazard into account. For more information, see the 7th edition of “Bretherick’s Handbook of Reactive Chemical Hazards”; Chem. Health Saf. 2001, DOI: 10.1016/s1074-9098(01)00247-7; and J. Chem. Educ. 1988, DOI: 10.1021/ed065pa226.

Just to be clear, these experimental results DO NOT demonstrate that secondary alcohols are peroxidizable. They demonstrate that isopropanol contaminated by a drying process may be peroxidizable, and speculate - but do not show - that the contaminant is isopropyl ether. Secondary alcohols do not belong in the same category of peroxidizables as, for example, the classic examples of diethyl ether and tetrahydrofuran.

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Authors (September 5, 2016 2:49 PM)

It was not among objectives of this work to elucidate all details of peroxides formation in isopropanol. All experiments were designed to answer simple yet important for chemical practice question "yes" or "no". And the answer is "no" for isopropanol itself (while "yes" for contaminants). We believe, it would be sensible to set up full size investigation on the subject only if the answer "yes" would be found for isopropanol.